tachyphylaxis
Tachyphylaxis is a fast fizzling-out: a drug works well on the first dose but loses punch after just a few closely spaced doses, sometimes within minutes. It is tolerance that develops almost in real time.
A classic mechanism involves drugs that act indirectly by releasing a stored transmitter. Indirectly-acting sympathomimetics such as ephedrine work by displacing noradrenaline from nerve-ending stores; once those stores are depleted by repeated dosing, the next dose has little left to release, so the effect collapses rapidly. In other cases tachyphylaxis reflects very rapid receptor desensitization or fast feedback adjustments.
The defining feature is the speed and the close spacing of doses, which sets tachyphylaxis apart from the slower tolerance that builds over days or weeks. Practically, simply giving more drug usually does not restore the effect; the system needs time to recover or refill, so spacing doses out is often the only fix.
Repeated doses of the decongestant ephedrine give progressively weaker responses as the noradrenaline stores it relies on run dry.
Rapid loss of effect on repeat dosing.
Tachyphylaxis is essentially a very rapid, acute form of tolerance; the boundary with desensitization is one of speed and scale rather than a fundamentally different process.